For example, the conductive polymer has such a high conductivity that it is used as an electrolyte of solid electrolyte capacitors such as in tantalum electrolyte capacitors, aluminum electrolyte capacitors, and niobium electrolyte capacitors.
As such conductive polymers in the application, used are ones obtained by applying chemical oxidation polymerization or electrolytic oxidation polymerization to polymeric monomers such as thiophene or its derivative, pyrrole or its derivative, and aniline or its derivative.
As a dopant, an organic sulfonic acid can be mainly used to carry out chemical oxidation polymerization of a polymerizable monomer such as thiophene or its derivative as mentioned above. In particular, it is said that an aromatic sulfonic acid is suitable. As an oxidant, a transition metal is used. In particular, it is said that ferric compound is suitable. Usually, a ferric salt of aromatic sulfonic acid can be used to serve as a dopant and oxidant upon the chemical oxidation polymerization of a polymerizable monomer such as thiophene or its derivative.
Among the ferric salts of aromatic sulfonic acid, it was reported as follows: Particularly useful are ferric toluenesulfonate, ferric methoxybenzene sulfonate and etc. In order to synthesize a conductive polymer using them, they are used as a dopant and oxidant and mixed with a polymerizable monomer such as thiophene or its derivative. These processes are simple and suitable for industrialization (see patent reference No. 1, and patent reference No. 2).
However, there is a problem such as inferior heat resistance and large leak current in case of the electrolyte capacitor prepared by using a conductive polymer obtained by using the ferric salts of aromatic sulfonic acid as an oxidant and dopant agent. This is considered because iron is not completely removed from the conductive polymer, the iron being used as an oxidant upon preparing the conductive polymer used as a solid electrolyte, and therefore, the iron remains in the conductive polymer.
Therefore, an oxidant of a non-iron salt type is considered. For example, it was reported to use one which has mixed ammonium persulfate aqueous solution with butylamine phenolsulfonate aqueous solution to serve as a solution of an oxidant and dopant agent (Patent Reference No. 3).
The electrolyte capacitor using a solid electrolyte of a conductive polymer prepared by using an oxidant and dopant agent of such a non-iron salt type has improved the heat resistance and reduced the leak current, compared with the electrolyte capacitor using a conductive polymer as a solid electrolyte prepared by using an oxidant and dopant agent of an iron salt type. However, the ammonium persulfate is deteriorated as time passes, so that unless such an oxidant and dopant agent solution is used immediately after it is prepared, it must be disposed of not only with ammonium persulfate that is cheap, but also butylamine phenolsulfonate that is expensive. Therefore, such an oxidant and dopant agent solution must have been prepared at the same time as the production of the electrolyte capacitor.
Then, in order to solve the problem associated with the preservation of the oxidant and dopant agent solution as explained above, it was reported to make a solution of a dopant such as 2-methylimidazole naphthalenesulfonate, separate from another solution of ammonium persulfate as an oxidant (Patent Reference No. 4).
However, in this case at the time of production of the electrolyte capacitor, 2-methylimidazole naphthalenesulfonate and ammonium persulfate react on the surface of a capacitor element to form a salt, which resulted in a lower capacity (capacitance) than expected since the dopant and the oxidant tend to be difficult to enter the inside of the capacitor element.